The present invention relates to image alignment in particle projection lithography. In particular, the invention relates to an apparatus in a particle projection lithography system for use in an alignment system adapted to measure the position and shape of an optical image of a pattern of structures formed in a mask and imaged onto a target by means of a broad particle beam, wherein the apparatus has a plurality of alignment marks adapted to produce secondary radiation upon irradiation with radiation of said particle beam and is provided with an aperture through which passes the part of the beam that produces the optical image.
Methods and devices for the alignment of the imaged structure pattern on a target, such as a wafer substrate, in a particle projection system are well-known and are discussed in detail in the U.S. Pat. Nos. 4,967,088 and 4,823,011. The entire contents of these two U.S. patent documents are hereby incorporated by reference.
Of the various processes required to manufacture semiconductor devices, lithography is highly important. Simply described, the lithography process begins by coating a substrate, e.g. a silicon wafer with a thin layer of a material, called photo-resist or simply xe2x80x9cresistxe2x80x9d, which is sensitive to radiation such as ion radiation. A lithographic exposure tool projects an image of a pattern contained on a mask or reticle onto the resist-coated wafer. The wafer may be stepped through a series of exposure positions by which the same pattern of the mask is exposed a number of times on the wafer, each exposure position corresponding to one of a number of chips into which the wafer will be parted after all processing. Development leaves a resist pattern that delineates the desired images on the wafer surface. The wafer is then subjected to any one of many possible processes such as etching, oxidation, ion implantation, diffusion, and deposition.
In the process of forming a pattern on a wafer to conform to a desired circuit design, it is often necessary to image several complementary mask patterns at a single chip position. To accurately produce the features of a given circuit, the various mask patterns must be carefully overlayed, and referencing of the image with the wafer position is necessary. The demands on quality of such overlay are becoming increasingly stringent from year to year as circuit features continue to become smaller, in accordance with the trend in the microelectronic field.
The alignment system disclosed in the U.S. Pat. Nos. 4,967,088 and 4,823,011 provides real time measurement and adjustment of the position and size of the image field before and during the time of exposure. The alignment is done, e.g., with respect to the X and Y translation of the image in the wafer plane, rotation angle xcex8 and magnification MX and MY with respect to the lateral scales, respectively, as well as trapezoid distortion xcex94X and xcex94Y. During exposure, variations of these parameters may occur by voltage fluctuations (magnification M) or by slight movements of the target station with respect to the projection system, both in lateral directions X,Y and in Z-direction, the latter causing also change in magnification M. All these parameters can be measured from a set of alignment marks provided on, e.g., a reference plate which is positioned in front of the target as seen in the direction of the irradiating beam. The optical image is projected through an aperture provided in the reference plate, while the alignment marks are positioned on the reference plate around the aperture and are irradiated by respective reference beams which are formed in the mask and imaged onto the plane of the reference plate together with the pattern beam used for the optical image of the structure pattern to be formed on the target.
In particular when ions are used as irradiating particles, the depth of focus can be very high, so that the reference beams which are focused on the target plane are still in focus sufficiently in the plane of the reference plate even if the reference plane is set off from the target by some distance, as long as this distance is less than the depth of focus, allowing to provide tools such as a beam shutter between the target and the alignment system. Moreover, a spatial separation of the substrate and the alignment system can be useful to avoid contamination of the alignment detectors by material scattered off from the substrate. If, however, the distance of the reference plane from the target is not negligible, the size and shape of the image on the target depend also on the angle at which different parts of the beam traverse the reference plane. If voltage fluctuations or Z shift of the target occur, it is therefore not sufficient to correct merely for the magnification, which is what known alignment systems do. Known alignment systems do not measure the angle at which the reference beam impinges on the reference plate and therefore do not allow for an alignment of the angle at which the beam is impinging onto the target plane. The consequence is a possible misalignment between the corrected image and desired shape in case of voltage fluctuations or target shift or tilt.
In is an aim of the present invention to offer a way for pattern alignment which allows the measurement and adjustment of beam properties that are dependent on the z-coordinate, that is, the position along the optical axis. In particular, the invention aims at the measurement and adjustment of the direction in which the beam traverses the alignment system.
This aim is met by an apparatus as mentioned in the beginning wherein, according to the invention, the apparatus has a plurality of alignment marks which are
positioned outside the aperture for the beam part generating the optical image of the structure pattern,
arranged at positions to coincide with particle reference beams projected through reference beam forming structures provided on the mask while said optical image is projected onto the target, and
situated on at least two different levels over the target as seen along the directions of the respective reference beams.
The provision of alignment marks at different Z levels allows for an alignment which takes into account a possible non-vanishing beam landing angle or other beam landing parameters. By virtue of the invention, alignment parameters, such as the lateral scale or magnification, can be determined for each level and adjusted to, e.g., preset nominal values.
By virtue of the comparatively large distance of levels the invention enables detection of even small differences in the alignment parameters occurring when varying the Z position along the optical axis. Thus, the invention offers the possibility to space apart the alignment plane from the target plane without loss of control of the image position and shape, and thus helps to relax the requirements posed on the optical system with respect to homocentricity or telecentricity of the beam projected onto the target.
In this context, it is suitable if the system has a high depth of focus (DOF) so that the region of focusing of the beam covers the target and alignment planes even when they are spaced apart considerably. For this, an ion projection lithography system wherein the particles are ions is particularly useful.
Preferably, in order to reliably maintain preset locations of the alignment marks according to the requirements of the projection system, the alignment marks are situated on positioning means disposed outside the aperture for the image generating beam part and are positioned on sides of the positioning means directed towards the mask. Preferably, the alignment marks may be formed on the surface of platelets, the platelets being fixed onto the positioning means by bonding. Moreover, the platelets may be silicon platelets in which the alignment marks are formed as structures of different height with respect to the surrounding surface of the silicon platelet, e.g. as grooves.
A preferred embodiment comprises a reference plate adapted to be positioned in a predetermined position relative to the target, being provided with an aperture corresponding in size to the part of the beam that generates said optical image, and with alignment marks disposed outside said aperture. Suitably, the reference plate may be a zerodur plate.
In order to obtain a compact arrangement of the alignment setup, the apparatus according to the invention is further provided with detector means for each alignment mark provided with an electrode adapted to measure a secondary electron current emitted from the alignment marks. In this case, the electrode may be formed asymmetric with respect to the corresponding alignment mark and held at a positive electrical potential with respect to the potential of the alignment mark and surrounding components, in order to achieve a high yield of the secondary electrons.
Preferably, eight alignment marks are positioned on a first level and four alignment marks are positioned on a second level.
The use of particle radiation, in particular ion radiation, allows a considerable distance between the individual alignment planes, up to several millimeters, e.g. 5 mm. In order to increase the reliability of the alignment parameters obtained according to the invention with respect to the projection onto the target plane, the distance between two different levels may suitably be chosen in the order of the distance of the alignment marks from the plane defined by the target surface.